Loud-speaker with pressure-equalized chamber system enclosing flux gap



Oct. 21, 1947. H. s. KNowLEs 2,429,470.

LOUD-SPEAKER WITH PRESSURE-EQUALIZED CHAMBER SYSTEM ENCLOSING FLUX GAP l original Filed June 4, 1934 s sheets-sheet 1 l l -L Oct. 21, 1947. H. s. KNowLEs 2,429,470 LOUD-SPEAKER WITH PRESSURE-EQUALIZED CHAMBER SYSTEM ENCLOSING IFLUXYGAP originai Filed June 4, -1954l As seets-/sneet \2 4 gf/zyjz aff 0Lz1,1947. H S';KNOWLES 2,429,470

. LOUD--SEAEAKER` WITH PRESSURE-EQUALIZED CHAMBER SYSTEM ENCLOSING FLUX GrAPl 'f Original Filed June 4, 1934 3 Sheets-Sheet 3 V l/fi Kiwa/e6? Patented Oct. 21, 1947 UNITED STATES vrn'rlazNT AFFICI?.

FLUX GAP Hugh S. Knowles, River Forest, Ill., assignor to Jensen Manufacturing Company, a corporation of Nevada Original Application June 4, 1934, Serial No.

728,993. Divided and this application September 3, 1942, Serial No. 457,159

7 Claims.

This invention is a division of my application y S. N. '728,993 led June 4, 1934, now Patent No. 2,295,483, granted Sept. 8, 1942, and relates to loud speakers, and is more particularly directed to loud speakers or telephones of the electro-dynamic or moving coil type.

A speaker of this type, when operating in open or unconned fluid, radiates little power at 10W frequencies, because the compression wave from ciently low to prevent `2 reactance high at'high frequencies. Thisvmay, of course, require theuse ofa large number of sections if all` the sections are'made uniform, in order to make the stiffness at low frequency sulfipo'wer loss of the sound radiating system.

In carrying out the object ofv my invention, which is to improve the emciency of loud speaker units by substantially eliminating the power loss one side of the moving system is almost completedue to the resistance component of the impedance 1y neutralized by the rarefaction Wave on the to motion of the uid within the restricted voice other side. The use of an infinite baille for sepacoil passages =I make the fluid partmle VQQC'fY rtm; the frort and 1rleait'ratdiaion of a movlil'ilg in these pasagies negtliigiblyll'g Whfl hebglbsj ap ragm sys em wi e ec ive y overcome t s ing system S n mo on. s 1s o e s difficulty, but such a construction is, of course, ,15 stantially elimlnatin'gany PreSSul dielenl'i not of practical utility. across these restricted passages.` Yaccomp s Also, there is considerable power loss or loss of this by so proportionng the enclosures communieifective sound radiation from a loud speaker due cating with these passages that the pressure in;- to the resistance caused .by forcing the air in and creases (or decreases) an equal amOllnt'in eah out through the annular gap between the top enclosure when the vibrating system 1s displaced, liglate ofdtletcoil asmbly and thovgice coil bobor by proflidilglowdmpedalle pgft tnsll'allel in, an e ween e voice coil o bin and the with the ig mpe ance Pa S S a core. At low frequencies, especially below the `tially all the iluid displacemeinti takes 513C# fundamental resonance of the system, the rethrough the low impedance an owresis ance si'stive component impedes the movement of the 25 paths, giving a vnegligible Dressur@ derential diaphragm with a corresponding decrease in efacross the high impedancehilths and therefore ective sound radiation, which may be termed a negligible fluid flowrthroug em. power loss, due to this highly resistive impedance. Another object secured by the m6591115 'Ulven- Also, the enclosure produces constructive and detion is the provision of a vibrationA cone orA diastructive interference at various frequencies, 3G phragm which possesses relativelygreat mechanicausing changes in the velocity of the moving cal strength tostatic deflection, and which is so system, or resonance. Particularly is this true formed as to havbeccentiizlilc radial sectors; florggg; when the enclosure is made large enough to procreasing or distr 'u ng e resonance a 1 vide low stiffness reactance at low frequencies in frequencies. This eccentricity may .be produced order to avoid any objectlonably high fundamenin any desiredmanner as will be pointed out heretal resonant frequency of the system. inafter. The eccentric rings, Corrugatlols off the One object of the present invention is the prolike, may be formed in the cone, or may e 0 mafvision of an acoustical network which, in itself, terial such as fibrous rings, whichadd mass, stiiT- is acoustically coupled, and which, as an entity, ness and resistance to the diaphragm. is coupled to a vibrating system, the network hav- 40 other objects and advantages of the present lllctgs raggfngst 0 the uehfiul fri' invention, such as certain details ofY construction o e 1 m Sys em' s ne and operation which present novel fea ures no work is designed to add but little stilfness to the to my knowledge, heretofore providedJ will be floim? Systen at 10W ffequelcles Wlth a rela 45 described in detail in connection with the foitlve y uicreasmg amount of Staines? as the fre' lowing description, which, taken in connection quency increases, so that the eliective reactance 1 .with the accompanying drawings, w1ll disc ose of the vibrating system is a minimum and the t th. kin th art referred embodi. velocity is a maximum in most of the useful freo ose s ed m e a p quency ranges. ment of the present invention. Y

This network is adapted to cooperate with the 5o In the drawings:

enclosure for reducing the mechanical resistance to flow through the air gap. It preferably comprises a plurality of tapered or, if desired, uniform iterated chambers, preferably having such stiffness in the iirst section as to make the stiffness Figure 1 is a sectional elevational view of a speaker unit embodying the teachings of the present invention.

Figure 2 is a vertical sectional view of a modifled form o1 the invention;

3 formed of wood, a lower wall 4 preferably formedvk of metal or a heat conducting-material, Vand, a

front Wall 5 formed ofmetal or a heat *conduct-- ing material, the front wall 5 being provided with the opening 6 disposed below th'e center of the enclosure for centeringthe speakerunit'with".

annular' outwardly `flaring of the respect thereto. An horn 1 is mounted about the defining edge opening 6 and is secured invposition by means of 15.

' 1 J f-dinarily results in some ow of fluid from the Spaces the annular angle bracket'meniber.

Within the enclosure irregularly shaped rigid dispersion plates r vanes, which may be made of 4 metal, wood, thin fibrous material, or any rigid dispersion surface which preferably occupies small volume so that the total eiective Volume of. the enclosure is not appreciably reduced. These surfaces are so shaped and mounted'that adjacent elements of area on the plates differ in distance from some element of area on the cone. Optionally, these vanes may beperforated or the surfaces of the vanes may be coated with material for damping their self-resonance.

To provide for such..changes Vin barometric pressure which otherwise might produce a differential pressure between the front and back sides of the diaphragm, a high resistance vent or tube I2 may be provided, .which extends through a suitable opening in. the rear wall 3 ofthe enclosure.

The speaker unit,vwhich, in this particular embodiment, is of the, large diaphragm electro-dynamic type, has a field structure I3, comprising a Vsubstantially cup-shaped housing, which is secured to a top plate I4, and has mounted therein the field coil I5- .ThiscoiL Wh'en energized, produces the desired magneticux in the annular air gap- I6. The. vibrating assembly consists of the diaphragm Il, the voice coil and bobbin assembly I8, the flexible centering member I9, the dome-shaped cover 20,;the flexible annular diaphragmsupport or-closure member 22 'and the self-dampingstrips23.

The rings-.or corrugatons. 2 4, since. they may comprise rings felted or ,otherwise mounted Von th'e diaphragm .or Acorru'gations vformed directly in` the diaphragm`are' l,preferably .so made that the annular ring sections of." the diaphragm lbetween them/are not oconst'ant width. The rings `may be disposed to llein the position as determined by the inter-section of theV surface of the diaphragm and a plane not quite perpendicular to the axis of the speaker. Th'e radial distance between diiierent sections of the rings and between the rings and the annular flexible support 22 should preferably'not be a constant.

The field-assembly I3 -is held in position by means of a pair Vofstrip supporting 'members 25 which are formed of heat conducting material, and whichY are spacedapart to provide a through vair .space therebetween invorder to-eliminate any resonating effects fromfthe provision of a confined chamber formed by these conducting strips. These .strips serve toconduct the heat generated in the coil I5 from the housing I3 to the metal wall 4 of the enclosure, and, in combination with the metallic cone housing 26 which .extends between the top plate I4 and theouter peripheral edge of the diaphragm Il, serves t@ Qlidllct I, I preferably providev heat away from the coil assembly and to the bottom and front walls 4 and 5 of the enclosure. The Wall members 2, 3 and 4 are provided with stiifening channels or ridges 21 formed therein, which, in addition, serve to preventY self-resonance of these wall surfaces.

i In the conventional type of speaker, the movement of the voiceicoil bobbin assembly I8 into and out of the'air gap I6 between the core indicated generally at 28 and the defining edge of the opening in the top plate I4 results in move- {,mentof the diaphragm Il. This produces an increase in the pressure of fluid on the back sides of the centering member I9 and the bobbin cover pas the diaphragm moves inwardly, which orback of the centering member and bobbin cover through the air gap between the top plate I4 and the Voice coil assembly IB, and between the voice coil assembly and the core tip 28 of the field structure.

With such an air flow, which directionally is out of phase with voice coil assembly movement. there is a considerable power loss due to frictional resistance of the air, which results in loss of effective sound radiating capacity, especially at low frequencies. In the present invention, the resistive component of the reaction of the-fluid on the moving system, due to unwanted movement of the fluid through this gap, is substantially eliminated.

The core 28 is preferably made hollow, and is closed at its outer end by means of the closure plate 29` which is substantially fluid-tight. Thus, when the voice coil assembly I8Wth the dome 20, moves inwardly with respect to the field assembly, there is compression rof'fiuid in the space 3D of the core assembly. At the same time, due to the volume of the defining end of the voice coil assembly, there is compression of fluid in the annular chamber' 32 surrounding the core tip 23, and sealed from the coil assembly I5 by means of the annular shield member 33. The volumes 30 and 32 are preferably so chosen that, with the proportionate displacement of the end of the voice coil assembly I8 and the displacement of the dome 20, the fluid is compressed by an equal amount in these two chambers so that there is no differential in pressure therebetween, which prevents the flow of fluid from the chamber 32 past the outer surface of the tip 28 and within the Voice coil assembly I8 into the space 30, or vice versa.

The flexible centering member I9, which is, in this embodiment of the invention, composed of an imperforate, eccentrically-corrugated flexible member, is substantially fluid-tight, being mounted upon a shoulder 34 formed on the spider Y26, by means of an annular channel member 35, which allows tightening of the outer defining edge ofthe member I9 into sealing engagement with the shoulder 34 without producing any torsional stresses therein. As the diaphragm, therefore, moves inwardly, the flexible member I9 compresses the air in the chamber 3l, which air may be allowed to escape to some extent through the ports 38 formed in the spider 25, and Which lead to an annular chamber 39 therein. Preferably, the volume of the chambers 3l and 39 with the space within the ports 38 is so proportioned with respect to the chambers 3U and 32 that upon inward movement of the vibrating assembly, the ,compression effected in the chamber 31, or in the chambers 31, 38 and 39, is substantially equal to the compression existing in the chambers 30 and 32, which means that the pressure at opposite ends of the gap passage 40 between the top plate I4 and the outer surface of the voice coil assemblyV I8 remains substantially equalized, so that no flow of uid through this gap is caused. This provides for inward movement of the voice coil assembly without producing a substantial ow of uid through the annular gap between the core and the top plate, which means that the resistive component or friction loss is eleminated, thus improving eiective sound radiation at low frequency. The pressures existing in the chambers 36, 32 and 31 (including 38 and 39) produce a, pure stiffness for the vibrating assembly, which stiffness is relatively low at low frequencies when the three chambers are relatively large so that the unit variation in pressure is small.

To further reduce the stiffness contributed by the volumes 30, 32 and 31, it may be desirable to substitute a substantially air-tight bulbous enclosure for the cover plate 29, in order to increase the total volume of the chamber 30. It is to be understood that these three volumes are not independently variable since the requirement is that the instantaneous pressure in all three be substantially the same at low frequencies. If desired, the entire volume of the enclosure may be used for one of the chambers, the other two volumes being proportioned in accordance therewith to produce the same intantaneous pressure in each of the chambers upon vibration of the system at low frequencies.

The cone housing 26 is centered on the top plate I4 by the shoulder 42, which shoulder is formed concentric with the gap and the voice coil assembly. It is important in an assembly of this type in which the voice coil is completely enclosed, to provide means for permitting access to the voice coil to permit repairs, adjustments, or the removal of foreign particles inadvertently introduced during assembly. The shielding member 33 fits snugly on the core, but may be removed from it and from the top plate i4 to permit the removal of particles which inadvertently get into the enclosure 32. This construction makes all parts of the enclosure readily accessible.

In this embodiment of the invention, the dome 20 is a relatively light rigid member which may have embossings thereon to increase its rigidity, vand is connected to the voice coil bobbin I8 by means of a stiff annular ring or shoulder which is centered on the bobbin and helps to keep the voice coil round. The stiffness of the ring and the mass of the dome are normally so chosen as to give a rise in velocity of the dome at high frequencies. This frequency may be slightly above the frequencies at which the sound output of the speaker without the dome would begin to fall. This extends the frequency range of the speaker.

The use of the imperforate spider or centering member I9 prevents paramagnetic dust particles or the like which may be suspended in the uid surrounding the magnetic circuit of the speaker from being drawn past the dome 20 around the outer periphery of the diaphragm l1 and into the annular gap about the core tip, since the introduction of such particles thereinto will destroy the operating eiciency of the speaker.

Considering now the embodiment of the invention shown in Figure 2, in which is provided a field coil assembly comprising the field housing 45, having the field coil 46 mounted therein about a centrally extending core 41, the eld housing being secured to the top plate 48 by bolt members 49 spaced about the periphery thereof. These bolts, at the same time, are adapted to secure the cone-supporting spider .56 to the top plate.

The top plate 48 is provided with an annular central opening 53, which is disposed concentrically about the core tip 54. A shielding member or the like, indicated at 55, is secured to the under surface of the top plate 48, and engages the outer periphery of the core 54 to define an annular chamber 56 therebetween, corresponding to the chamber 32 of Figure 1. The core 41 is provided with a tapered hollow opening, which'is closed at its outer end by the closure plate 51 bolted to the housing 45, and at its inner end is provided with an annular retaining member 58, for a purpose to be hereinafter described.

Within'the hollow core 41 is disposed'what I term an acoustically coupled network, winch, in the present embodiment, comprises three successive chambers, indicated at 59, 60 and 6I, which are separated by partitions 62 and 63 and communicate by means of centrally disposed tubes 64, 65 and 66 with the space 51 behind the dome 68 of the diaphragm. If desired, the chamber 59 may be iilled with sound absorbing material, although this is not necessary. This material may comprise hair felt, mineral or glass wool, or the equivalent, to provide a resistive termination at high frequency. In the particular form shown, the masses and stiifnesses of successive sections of the lter are unequal so that a tapered transmission line is produced. 'I'he member 53 holds this assembly in position within the hollow core 41. The chamber 61, between the dome 68, Ythe retaining member 58, the partition 69 and the tube 66 provides a stiffness reactance to the diaphragm through the dome 68. This is connected by the mass of the fluid in the tube 66 to the volume or chamber 6I included between the partitions 63 and 69, which chamber addsa certain stiffness reactance, being in turn coupled through the'mass of the uid in tube 65 to the space or volume 60 between the partitions 63 and 69 and outside of the tubes 64 and 65. The latter chamber 60 is connected through the tube 64 to the chamber 59 to provide the next successive stiffness reactance to the vibrating system.

In general, the fundamental resonance frequency of the vibrating sound radiating system is made low enough to prevent introduction of objectionable resonance in the voice range. It is therefore desirable to couple the vibrating system to a network which produces very little stilf ness at low frequencies and increasing stiffness at higher frequencies. The velocity of the system and hence the sound output, is increased by the addition of sound reactances, since this lowers the impedance of the vibrating system. This network may either be a mechanical or acoustical network. In this particular embodiment an acoustical network is employed.

In the particular embodiment shown, at ex- .tremely low frequencies the volumes 59, 60, 6| and 61 areall in parallel, that is, when the dome 68 moves in lslowly the mass reactance of the fluid in the tubes 64, 65 and 66 may be neglected so that the fluid in the entire network is compressed. This volume is so chosen that, for a given displacementvof the dome 58, the voice coil 16, the diaphragm 52 and the centering member 15, which constitute the moving system, the pressure in chamber 61 just equals the pressure inthe chamber 56, which in turn equals the pressure inthe chamber l12', ports 'I3 and annular chamber 14so that there is no differential in pressure through either the annular gap between the voice coil and the top plate 48, or between the voice coil and the core tip 54. It is therefore apparent that a substantially pure reactance or stiffness component is added to the system at low frequencies.

At increasingly higher frequencies, the mass reactance of the uid in the tube 66 increases s0. that the particle velocity of the fluid is smaller, and the compression becomes largely localized in chamber 61. This produces a stiifness which is greater than that of the low frequency stiffness and which is approximately inversely in ratio as the volume of the space 61 is to the combined volumes 59, 60, 6| and 61.

In general, since it is desirable to make the ratio of these stiffnesses large in order to make the stiffness reactance large at high frequencies where the mass reactance of the vibrating system is large, it is desirable that the volume shown at 61 be substantially smaller than the remaining volumes. This volume may be controlled by movement of the partition 69 toward the dome 68, or it may be formed as an integral part of the retaining washer 58. Thus, the volume 61 may be reduced to the minimum required by the maximum displacement of the vibrating system. The number of sections may be increased, if desired, by successively reducing the volumes 58, 68 and 6|, and by th'e addition of additional partitions and connecting tubes. By thus increasing the number of sections and the length of the network or line, the ratio of the stiffness at low and at very high frequencies can be increased considerably.

In the embodiment shown in Figure 2 the tubes 13 perform the same coupling function between volumes 12 and 14 that the tubes 64, 65 and 66 provide between volumes 59, 60, 6| and 61.

Referring now to the embodiment sh'own in Figure 3, an alternative construction is provided in which a plurality of pairs of relatively thin flexible members |60 and |6| form support means to which is cemented a thin perforate screen |62, which may be made of porous cloth, felt, or similar material, or formed as a light metal screen. In situations in which no other provision is made for the by-passing of the high resistance fluid iiow path between the voice coil |63 and the top plate |64, it is necessary that the screen |62 be perforated.

On the other hand, if any of the arrangements previously described are used in which no venting through the flexible member is necessary, this material should preferably consist of` a flexible imperforate member, such as rubber, leather, thin sheet metal or the like, which has a slight amount of elasticity and a large amount of mechanical resistance to suppress the natural modes or vibrations of the flexible supporting arms |60 and |6|. Th'ese supporting arms may either be of the conventional flat radial or non-radial type.

As an alternative, in place of the screen |62, the centering and enclosing functions may be separated by the use of a bellows-like collar |65 which preferably is corrugated eccentrically, having corrugations which vary in depth and width at different parts of its circumference to supress the natural modes of vibration. The same precautions should be exercised in its design and construction that are exercised in the case of the flexible support members. It may be perforated in a case in which the high impedance 'fluid paths are to be shunted through thismemben, or it may be imperforate incases in which the effector the high impedance, path is eliminated by eliminating the pressure differential as described in connection with Figure 1. v

The flexible centering means is supported upon a ring member |66 which' is fastened to the top plate |64 by means of screws |61. These screws pass through clearance holes in the top plate so that the voice coil may be centered in the gap, after which the screws |61 are tightened. AThe. flexible supporting members |60 and |6| are clamped by the circular channel or clamping ring |68, and the space between the screen |62 and the top plate, indicated at |69, is vented through the ports |10 formed in the supporting ring |66. This volume is also vented through the top plate by means of ports |12 formed there-1y in, which preferably are covered by screen members |13.

The volume or chamber |14, beneath the dome |15, which dome is preferably provided with reenforcing ribs |16, similar to the ribs shown in Figure 2, is vented through the diagonal ports |11 and through the dome-shaped depression |18, the depression |18 communicating through ports |19 with the annular chamber |80 formed by the closure member |82. This chamber |80 is vented through ports |83 and screens or filter members |84 into the space occupied by the field coil |85.

By the provision of the depression |18, the volume under the dome |15 is considerably increased, and the pressure variations are therefore correspondingly reduced. As far as possible, the construction at the core tip is formed to prevent increasing the reluctance of the core tip. 'I'he diagonal vents I 11 in the voice coil bobbin |86 are so formed that the intermediate supports between the ports |11 act as the stiffness members through which the voice coil drives the cone |81. By increasing the angle subtended by the strips with the axis of the coil, the stiffness can be reduced, and therefore the angularity of the openings |11 will, to a large extent, determine the selection of the stiness produced by the voice coil assembly and thus will control the mass of the voice coil winding and the effective mass of the cone at high frequencies, so that the sound radiated at high frequencies can be increased or decreased as desired. It will be noted that the dome |15, with its reenforcing webs |16, is provided with a shoulder |88 which cooperates with a corresponding shoulder |89 of the voice coil bobbin |86 to bear directly against the diaphragm |81. The dome |15 is thus provided with enclosed reenforcing members and with a shoulder which bears directly on the voice coil bobbin shoulder and provides good thermal conduction to the voice coil bobbin |86 and diaphragm |81. The dome is preferably formed, in each of the embodiments of the invention described, of some light weight metal having high heat conductivity, such as aluminum, magnesium beryllium and their alloys.

What is claimedis:

l. In a loud speaker, eld magnet structure forming an annular fluid gap, a vibra-ting assembly comprising a diaphragm, a bobbin closed at its outer end and carrying a voice coil, and a closure member attached'to the outer surface of the diaphragm near the small end thereof, means including the inner end of saidbobbin forming a chamber located within said magnet structure and communicating with said fluid gap, means including said closure member forming a chamber located without said magnet structure and communicating with said rst-mentioned chamber through said gap, and means including said bobbin forming a chamber located without said magnet structure and communicating with said first-mentioned chamber through said gap, each of said three chambers being of such volume and so related to the volumes of the other two chambers that the instantaneous proportionate volumetric displacement within each chamber resulting from movement of the vibrating assembly is the same in all chambers.

2. In a loud speaker, field magnet structure forming an annular iiuid gap, a vibrating assembly comprising a diaphragm, a closure member attached to the outer surface of the diaphragm near the small end thereof, and a bobbin centered within said gap and dividing said gap into an inner annular passageway and an outer annular passageway, said bobbin being closed and attached at its outer end to the small end of the diaphragm and carrying a voice coil, means including the inner end of said bobbin forming an internal chamber located within said magnet structure and communicating with both the inner and outer annular pasageways, means including said closure member forming a first external chamber located without said magnet structure and communicating with said internal chamber through the outer annular passageway, and means including said bobbin forming a second external chamber located without said magnet structure and communicating with said internal chamber through the inner annular passageway, the volumes of said three chambers being such and so related that instantaneous pressures in all chambers are always equal whereby the pressure differentials at the ends of both annular passageways are substantially nil.

3. In a loud speaker, field magnet structure comprising a top plate having a central opening and a central pole piece extending through said opening to form a fluid gap, said central pole piece having a well formed therein opening at the exposed tip thereof, a vibrating assembly comprising a diaphragm, a bobbin closed at its outer end and carrying a voice coil, and a closure member attached to the outer surface of the diaphragm near the small end thereof, means including the inner end of said bobbin forming a chamber located within said magnet structure and communicating with said fluid gap, means including said closure member forming a chamber located without said magnet structure and communicating with said first-mentioned charnber through said gap, and means including said bobbin forming a chamber located without said magnet structure and communicating with said rst-mentioned chamber through said gap, said last-mentioned chamber including the Volume of the well within said central pole piece, the Volumes of said three chambers being such and so related that instantaneous pressure variations resulting from movement of the Vibrating assembly are the same in all chambers.

4. The structure of claim 1 and including a network of acoustic reactance chambers coupled with and forming a part of the volume of at least one of the two chambers without the eld magnet structure.

5. The structure of claim 3 and including at least one transverse partition within the well of the central pole piece forming at least two acoustic reactance chambers and a tube segment passing through the partition to acoustically couple adjacent reactance chambers.

6. The structure of claim 3 in which communication between the volume within the bobbin and the volume within the well in said central pole pieces is restricted to a relatively long and narrow passageway.

7. In a loud speaker, eld magnet structure forming an annular fluid gap, a Vibrating assembly comprising a diaphragm, a bobbin closed at its outer end and carrying a voice coil, an open spider centering member attached to said diaphragm at the small end thereof, and a flexible closure member attached to the outer surface of the diaphragm at a small distance from the small end thereof, means including the inner end of said bobbin forming a chamber located within said magnet structure and communicating with said fluid gap, means including said bobbin forming a chamber located without said magnet structure and communicating with said first-mentioned chamber through said gap, and means including said flexible closure member forming a chamber located without said magnet structure and communicating with said first-mentioned chamber through said gap, said centering member being disposed within said last-mentioned chamber, each of said three chambers being of such volume and so related to the volumes of the other two chambers that the instantaneous proportionate volumetric displacement within each chamber resulting from movement of the vibrating assembly is the same in all chambers.

HUGH S. KNOWLES.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,876,831 Ballentine Sept. 13, 1932l 1,923,965 Brennan Aug. 22, 1933 1,964,606 Thuras June 26, 1934 2,037,187 Wente Apr. 14, 1936 2,041,157 Thuras May 19, 1936 2,084,944 Cornwell June 22, 1937 2,086,649 Thuras July 13, 1937 2,095,373 Speer Oct. 12, 1937 v 2,252,846 Giannini Aug. 14, 1941 2,261,110 Engholm Nov. 4, 1941 2,261,111 Engholm Nov. 4, 1941 

